The present invention relates to liquid crystal displays, and particularly to a backlight device for liquid crystal displays.
The technology of liquid crystal display, particularly color liquid crystal display, has been remarkably progressed in recent years. There are now many different liquid crystal displays of which the display quality is as excellent as CRT. In addition, notebook-type personal computers have been widely used which have always backlight devices incorporated as illuminators. The backlight device is indispensable to the direct viewing type color liquid crystal display.
The color liquid crystal displays can be roughly classified into two types: the TN (twisted nematic) liquid crystal display of active matrix drive using TFT (thin film transistor) and the STN (super-twisted nematic) liquid crystal display of multiplex drive. Either one of these types has a liquid crystal layer held by glass substrates, and polarizing plates that are disposed on both sides of crystal layer to modulate the polarized state of linearly polarized incident light. The backlight devices for these liquid crystal displays have various luminance levels depending on the use. Particularly in the color notebook type personal computers, small thickness, light weight and low power consumption are absolutely necessary as well as the required brightness.
In the conventional liquid crystal displays, since the light emitted from the backlight device that is disposed on the back of the liquid crystal display is non-polarized light, more than half of the incident light to the display is absorbed by the polarizing plate provided on the incident side of either one of TN and STN type displays, that is, the light utilization efficiency is low and thus displaying is dark. Therefore, for bright displaying, the power consumption must be increased.
To solve these problems, there is proposed a backlight device which emits polarized light as disclosed in, for example, Japanese Patent Application JP-A-6-265892. This backlight device has polarizing means provided on the light emitting side of a plane type waveguide for the emitted light to be substantially perpendicular to the surface of the plane type waveguide, and a polarizing beam splitter provided on the polarizing means. The polarizing beam splitter has polarizing beam splitting layers laminated on a column-shaped prism array with a triangular cross-section.
In order to achieve a high-performance, polarized light source capable of highly polarizing, it is necessary to make highly parallel light incident to the polarizing beam splitting layers. This requirement is satisfied by a proposed backlight device which has a thin waveguide pipe provided nearby and has a microprism structure as disclosed in Japanese Patent Application JP-A-6-202107.
However, it is very difficult to simultaneously achieve a highly-parallel light source and a high degree of light uniformity in a plane. Any light uniformity in a plane is not described in this gazette. In addition, the polarizing beam splitter (or retroreflective polarizing sheet) using a dielectric multilayer needs lamination of multiple layers for high degree of polarization, and the multilayer-thickness control is severe, thus leading to high cost.